In recent years, seismic anisotropy has become an increasingly important topic in geophysics. Seismic anisotropy refers to a directional dependence of seismic wave properties (e.g., velocity) in rock medium. When seismic waves propagate during seismic data acquisition, velocity of the waves may be faster in one direction while slower in another direction. This directional variation can indicate certain structures in the rock that are on the scale of the seismic wavelength. Rocks can exhibit anisotropic properties for a number of reasons including, but not limited to, the presence of fractures, preferred orientation of mineral grains, shape of isotropic minerals, and thin bedding of isotropic layers. In particularly, fracture characterization is a key attribute in optimizing hydrocarbon recovery. Understanding fracture distribution and orientation helps identify sweet spots, especially for unconventional resources and shale plays.
Seismic data can provide indirect measure of fracture attributes. Velocity variation with azimuth (VVAZ) is a fracture detection method used to characterize fractures by measuring velocity response of seismic waves against varying azimuth (e.g., variation of stacking velocities with source-receiver azimuth). Typically, VVAZ uses prestack primary wave (P-wave) travel times from various azimuths and offsets to reconstruct normal moveout (NMO) velocity ellipse along the time dimension. Long axis of the NMO velocity ellipse corresponds to the P-wave traveling along a fracture strike direction (fast P-wave velocity) while the short axis corresponds to the P-wave traveling perpendicular the fracture strike direction (slow P-wave velocity). Some studies have suggested that relative ratio of the long and short axes of the ellipse is proportional to fracture density while fracture orientation is related to rotation of the ellipse. (Grechka and Tsvankin, 1998, Jenner, 2010, and Stein et al., 2010).
Conventional techniques for estimating seismic anisotropy can suffer from one or more technical challenges. For example, seismic anisotropy estimation can be expensive, labor-consuming and/or computationally-intensive, particularly when determining azimuthal velocities in all three dimensions of prestack seismic data. In addition, fracture-attribute estimations also tend to be less reliable when prestack seismic data have significant coherent and background noise. In general, an improper understanding of seismic anisotropy can lead to a number of issues including incorrect positions of exploration targets and improper estimations of petroleum reservoir.
Recently, a target-oriented VVAZ method (Chiu et al, 2012) was proposed, which directly inverted horizontal transverse isotropy (HTI) magnitude and orientation in a fracture layer. This target-oriented VVAZ method requires picking travel times between the top and base of the fracture layer. The advantage of this method minimizes the overburden effects and produces more accurate fracture estimations. Use of differential travel times between the top and the bottom of the target may lead to a more robust inversion result, even for a relatively thin fracture layer.